This invention concerns a photodetector for converting light signals into electrical signals.
There is an increasing demand for photodetectors able to detect efficiently optical signals modulated at frequencies above 40 GHz for applications such as millimetre-wave over optical fibre communication, high data rate optical networking, millimetre-wave and THz signal generation and radio-astronomy.
Conventionally, two main approaches for high speed photodetectors have emerged. One is to match the optical velocity and the electrical velocity in a waveguide photodiode structure in order to overcome the frequency response limitation arising from the device capacitance. Such a travelling wave (TW) structure offers a 3 dB bandwidth of 50 GHz with a responsivity of 0.2 A/W. The second approach is the use of an electron-only transfer structure because the electron transfer is faster than that for holes. An example of this approach is the Uni-Travelling Carrier structures (UTC) in which the electrons act as the only active carriers and determine the photoresponse. This UTC structure allows a 3 dB bandwidth of as high as 310 GHz with 0.07 A/W responsivity. The two techniques have also been combined by coupling a number of individual UTC photodiodes to an optical waveguide, their spacing and electrical interconnection being adjusted in an attempt to match the optical velocity along the waveguide to the electrical velocity of signals travelling along such interconnection, achieving a 115 GHz 3 dB bandwidth and 0.075 A/W responsivity.
However, in realising a high-speed photodetector, there are a number of competing requirements. In a waveguide photodetector, the absorption length for greater than 90% absorption in an absorber such as InGaAs is 3 μm. With a typical waveguide width of 4 μm to 6 μm wide, absorption in such a small area imposes a limit on the saturation power of the photodetector. The natural answer would be to increase the area of the absorber, e.g. by increasing the length of the waveguide. However, any depletion photodetector has an associated electrical capacitance, also known as depletion capacitance. The larger the junction area, the greater the capacitance. With a given load resistance, increased capacitance leads to proportionately lower device bandwidth. For bandwidths below 300 GHz the interaction length with the absorption layer in a waveguide photodetector could be kept sufficiently short (e.g. 10 μm) to have a low parasitic capacitance and still offer adequate responsivity and saturation power (but the maximum length will be limited by the parasitic capacitance of the device). However to obtain higher responsivity and saturation power a longer waveguide absorption section will be required thus increasing substantially the parasitic capacitance of the detector which will strongly limit its bandwidth.
The present invention seeks to alleviate the problem of simultaneously providing high bandwidth, high responsivity and high saturation power in a photodetector.
According to one aspect of the invention there is provided a photodetector comprising:
an active waveguide comprising an absorber for converting photons conveying an optical signal into charge carriers conveying a corresponding electrical signal;
a carrier collection layer for transporting the charge carriers conveying the electrical signal; and
a secondary waveguide immediately adjacent to the carrier collection layer, which supports fewer than 5 modes, for receiving the photons conveying the optical signal, and which is evanescently coupled to the active waveguide.
According to a preferred aspect of the invention, fast transport of said charge carriers away from said active waveguide is enabled, for example by using a uni-travelling carrier (UTC) structure.
According to another preferred aspect of the invention the photodetector comprises a travelling wave structure comprising a further waveguide arranged such that the phase velocity of the electrical signal along the further waveguide is substantially matched to the phase velocity of the optical signal in the active waveguide.
The photodetector according to the above aspects of the invention incorporates an extra waveguide in the structure, referred to as the secondary waveguide or equivalently as the passive waveguide. This additional passive waveguide allows for evanescent coupling with the active absorber waveguide, and the design also enables one or both of fast carrier transfer (like in UTC structure) and a travelling wave (TW) structure to be achieved. A travelling wave structure is a structure such that the optical signal travels at a velocity (phase velocity) comparable to that of the electrical signal along the waveguide/electrode.
The secondary waveguide receives the light to be detected, which light may be equivalently referred to herein as photons or electromagnetic radiation. The terms “light” or “photons” used herein do not imply limitation to any particular part of the electromagnetic spectrum, for example the terms are not limited to the visible part of the spectrum, and they explicitly include infrared radiation, near-infrared radiation, mid-infrared radiation, far-infrared radiation, terahertz wave radiation (THz wave) and millimetre wave radiation. The evanescent coupling enabled by the extra waveguide allows for higher saturation power because it increases the length over which absorption takes place, and also increases the responsivity of the detector.
Depending on the parameters of the structure, the evanescent coupling could imply a relatively long waveguide photodetector, and thus a relatively high parasitic capacitance. However, according to one aspect of the invention, the fact that the secondary waveguide is immediately adjacent to the depletion or carrier collection layer means that this can be overcome by matching the optical and electrical phase velocity (Travelling Wave technique). Effectively the capacitance is made part of a transmission line (further waveguide) formed by the capacitance per unit length of the detector and the inductance per unit length of the electrodes. If the travelling wave structure is well designed the main bandwidth limitation of the detector will be the electron transit time through the absorber (active waveguide) and not the capacitance of the device. The use of travelling wave (TW) techniques with evanescent coupling from a secondary waveguide allows the thickness of the absorber, which determines the electron transit time, to be reduced, thus increasing the bandwidth further.
Optionally the secondary waveguide is designed with dimensions so as to support only a few modes such that the detector can realise velocity matched travelling wave operation, so as to avoid the capacitance limitation, in order to reach the highest bandwidth possible while retaining compatibility with the added layer in the structure. For example the secondary waveguide may be single-moded or slightly multi-moded, such as supporting 2 or 3 modes, while remaining within the scheme of an evanescently coupled device.
The photodetector according to another aspect of the invention is designed to enhance the intrinsic bandwidth of the material by using structure-optimised fast carrier travel (such as a Uni Travelling Carrier structure, for example utilising only electron transport across the depletion or carrier collection region) thus offering a shorter transit time, i.e. charge carriers generated in the absorption layer are rapidly transported away.
According to a further aspect of the invention, mode converters are used to couple light more efficiently from a fibre into the secondary passive waveguide. The device according to the invention has the advantage that the coupling can be optimised without changing the parameters of the active part of the photodetector because of the use of the secondary passive waveguide. A polarisation-independent detector can also be fabricated through the use of an appropriately designed mode converter.